1. Field of the Invention
The present invention relates to a gas analyzer for measuring a gas (hereinafter referred to as object gas) containing a certain component (hereinafter referred to as object component) such as NOx including a combined oxygen, and a method for calibrating the gas analyzer.
2. Description of the Related Art
In the past, various methods and apparatus had been suggested which were used to measure the concentration of an object component contained in an object gas. For example, as a method for measuring NOx contained in an object gas such as a combustible gas, there has been known a process which involves the use of a gas sensor manufactured by forming Pt electrode and Rh electrode on an oxygen ion conductive solid electrolytic material such as zirconia, and which achieves the desired measurement by making use of a reducibility which Ph can provide for reducing NOx and by generating an electro motive force between two electrodes. However, such a gas sensor has been found to have the following disadvantages. Namely, if an oxygen concentration of an object gas (which might be a combustible gas) varies, such a variation will cause not only a significant change in the electro motive force, but also a trouble that the electromotive force will change only very slightly with respect to a change in NOx concentration. As a result, it is difficult to complete the desired measurement without being influenced by the above troubles.
Another disadvantage associated with the above mentioned prior art may be concluded as follows. Namely, in order to reduce the NOx component, a reductive gas such as CO is indispensable. However, under a condition where a large amount of NOx is generated and hence the fuel amount becomes extremely small, an amount of CO generated will become less than an amount of NOx generated. As a result, using a combustible gas formed under such a combustion condition has been found unable to perform the desired measurement.
Further, JP-A-63-38154 and 64-39545 have proposed that a series of electrochemical pump cells and sensor cells formed by Pt electrode and an oxygen ion conductive solid electrolytic material can be combined with another series of electro-chemical pump cells and sensor cells formed by Rh electrode and an oxygen ion conductive solid electrolytic material, so that NOx concentration may be measured by making use of differences between the electric current values of various pump cells.
Moreover, JP-A-1-277751 and JP-A-2-1543 have proposed a method comprising the steps of preparing a pair of electro-chemical pump cells and a pair of sensor cells, measuring a critical pump current under an oxygen partial pressure wherein NOx is not reduced with a sensor comprising one set out of two sets of pairs of the chemical pump cells and the sensor cells, and measuring a critical pump current under an oxygen partial pressure wherein NOx is reduced with another sensor which is the other pair of the pump cell and the sensor cell, then calculating a difference between the two critical pump currents; or employing a sensor comprising a pair of a pump cell and a sensor cell, and measuring a difference in the critical current by measuring a critical current by changing an oxygen partial pressure of an object gas between the cases wherein NOx is not reduced and wherein NOx is reduced.
However, when using the above described method for measuring NOx, most amount of a critical current value is occupied by an electric current generated by virtue of a large amount of oxygen contained in an object gas. Since an electric current value based on an object component NOx becomes extremely small, a difference between two large current values can be used to calculate only a small current value corresponding to the concentration of the object component NOx. For this reason, there has been a problem that when a change-over is conducted between one pair of sensors, it is impossible to perform a continuous measurement, or the measurement has only a decreased responsibility and a decreased precision. On the other hand, in the case of using two pairs of sensors, if an oxygen concentration of an object gas has a significant change, an error is likely to occur in a measured value. As a result, if an oxygen concentration of an object gas has a significant change, it is not allowed to use the above method disclosed in the above prior arts. This is because a dependency of a pump current on the oxygen concentration on one sensor will be different from a dependency of a pump current on the oxygen concentration on the other. In addition, when a difference occurs between two pairs of sensors with the elapse of time, such a difference will become an error, causing a problem that these sensors can not be used for a long time.
In view of the above, it has been made clear that an amount of oxygen existing in an object gas is responsible for a decreased measurement precision when measuring NOx or other object components.
In order to solve the problems described in the above, a further method has been proposed by JP-A-8-271476. According to this method, a first electro-chemical pump cell and a second electro-chemical pump cell are arranged in series so as to measure an object gas component (containing a combined oxygen) such as NOx contained in an object gas, and such a measurement can be performed continuously for a long time with a high responsibility, without having to be influenced by an oxygen concentration of the object gas. Further, according to this publication, an object gas containing an object component including a combined oxygen is introduced from an outside space into a first treatment zone under a predetermined diffusion resistance. Then, an amount of oxygen contained in an atmosphere within this zone is controlled in a first electro-chemical pump cell in the first treatment zone to a low partial pressure value which does not bring about any unfavourable influence to a measuring process to be carried out in a second treatment zone. On the other hand, in the second treatment zone, the object component contained in the atmosphere introduced from the first treatment zone is reduced or decomposed, an amount of oxygen generated at this moment is drawn out by virtue of an oxygen pumping action of a second electro-chemical pump cell. Subsequently, a pump current flowing into the second electro-chemical pump cell is detected which is then used to calculate an amount of object component contained in the object gas. However, even when using this improved method, there is still a problem, i.e., if the concentration of an oxygen contained in the object gas is high, such a high oxygen concentration will make it difficult to perform a correct measurement.
In order to solve the above problem, a still further method has been proposed by JP-A-9-113484 and JP-A-10-73563. According. to this method, a first electro-chemical pump cell, a second electro-chemical pump cell, and a third electro-chemical pump cell are arranged in series to measure an object gas component (containing a combined oxygen) such as NOx contained in an object gas, and such a measurement can be performed continuously for a long time with a high responsibility, without having to be influenced by an oxygen concentration of the object gas. Further, according to these publications, an object gas containing an object component including a combined oxygen is introduced from an outside space into a first treatment zone and a second treatment zone successively under a predetermined diffusion resistance. Then, an oxygen partial pressure of an atmosphere within this zone is controlled in a first electro-chemical pump cell in the first treatment zone to a low value which is sufficient to control the oxygen partial pressure in a second treatment zone. Subsequently, in the second treatment zone, the oxygen partial pressure of the atmosphere in the second treatment zone is controlled to a predetermined value by virtue of an oxygen pumping action effected by the second electro-chemical pump cell. Alternatively, in the first treatment zone, after the oxygen partial pressure of the atmosphere in the first treatment zone is controlled to a predetermined value by virtue of an oxygen pumping action effected by the first electro-chemical pump cell, the partial pressure is further controlled to a lower value which does not bring about any unfavorable influence to the measurement of the object component in the second treatment zone. Afterwards, in the third treatment zone, the object component of the atmosphere introduced from the second treatment zone is reduced or decomposed, an amount of oxygen generated at this moment is drawn out by virtue of an oxygen pumping action of a third electro-chemical pump cell. Therefore, a pump current flowing into the third electro-chemical pump cell may be detected which is then used to calculate an amount of object component contained in the object gas.
However, when using the sensors disclosed in the above two patent application publications, if an air ratio is xcex less than 1, an electric current Ip3 corresponding to NOx concentration at this time will decrease as shown in FIG. 5. Further, it is understood that this phenomenon is particularly remarkable in a weak rich area where the air ratio xcex is close to 1. This is because when the air ratio is xcexxe2x89xa71, some coexisting combustible gases such as carbon monoxide, hydrocarbons and hydrogen gas will react with oxygen coexisting therein. In contrast, if the air ratio is xcex less than 1, since an amount of the coexisting oxygen is not sufficient for the above combustion, the combustible gases will react with NOx which is an object gas component. As a result, the NOx component will disappear.
For this reason, the above described conventional methods and apparatus can not be used in the case where an object gas contains combustible gases such as carbon monoxide, hydrocarbons and hydrogen gas with a high concentration and where the air ratio is likely to become xcex less than 1. Particularly, with regard to a gasoline engine in which the air ratio xcex varies in the vicinity of 1 and with regard to a lean-burn engine which produces periodic spikes, since it is impossible to avoid a situation in which the air ratio xcex is likely to become less than 1, there has not been an appropriate method capable of effectively monitoring a discharged NOx.
Accordingly, it is an object of the present invention to provide an improved gas analyzer capable of correctly monitoring the concentration of a discharged NOx even if an air ratio xcex is less than 1. It is another object of the present invention to provide a gas analyzer calibrating method involving the use of the improved gas analyzer.
Namely, according to the present invention there is provided a gas analyzer comprising: a gas sensor which includes a first diffusion rate controlling passage, a first internal space communicated with the first diffusion rate controlling passage, a second diffusion rate controlling passage, a second internal space communicated with the second diffusion rate controlling passage, a third diffusion rate controlling passage, a third internal space communicated with the third diffusion rate controlling passage, and an air introducing duct; wherein the first diffusion rate controlling passage is a passage provided for introducing an object gas containing an object component including a sort of combined oxygen from an external gas existing space to the first internal space under a predetermined diffusion resistance; wherein the first internal space is provided for effecting the combustion of some combustible gases, and is provided with a first electro-chemical pump for adjusting an oxygen partial pressure within the first internal space, so that the internal space constantly contains a sufficient amount of oxygen capable of effecting the combustion of the combustible gases contained in the object gas which has been introduced into the internal space through the first diffusion rate controlling passage; wherein the second diffusion rate controlling passage is a passage provided for introducing the object gas treated in the first internal space to the second internal space under a predetermined diffusion resistance; wherein the second internal space is provided with a second electro-chemical pump which is so constructed that when an amount of oxygen is drawn from the second internal space atmosphere consisting of the object gas introduced into the second internal space through the second diffusion rate controlling passage, the oxygen partial pressure is decreased to a sufficiently low value which does not reduce or decompose the object gas and which is low enough for controlling the oxygen partial pressure in a third treatment zone; wherein the third diffusion rate controlling passage is a passage provided for introducing the object gas treated in the second internal space to the third internal space under a predetermined diffusion resistance; wherein the third internal space is provided with a third electro-chemical pump next to the third diffusion rage controlling passage and a fourth electro-chemical pump next to third electro-chemical pump, the third electro-chemical pump is so constructed that when the oxygen partial pressure of the atmosphere within the second internal space has a value which will not substantially reduce or decompose the object gas, the oxygen partial pressure is controlled to a further lower value which does not bring about any significant influence to the measurement of an amount of an object gas component, whereas the fourth electro-chemical pump is provided to reduce or decompose the object gas component introduced from the second internal space and to draw out an amount of oxygen generated at this moment; wherein the air introducing duct is provided in a manner such that the outside pump electrodes of the first and second electro-chemical pumps are isolated so that these electrodes are not directly exposed to the object gas and that this duct can serve as oxygen sources when oxygen is introduced into the first internal space; the gas analyzer further comprising: an operating section for operating the electro-chemical pumps provided in the first to third internal spaces of the gas sensor; a calculating section for performing a predetermined calculation on pumping currents flowing into the electro-chemical pumps so as to obtain a concentration value of the object gas component; a displaying/outputting section for displaying a value calculated in the calculating section or for outputting the value as an electric output; and a heater operating section for heating the gas sensor to a predetermined temperature.
Further, according to the present invention, a pumping current in the second treatment zone, a pumping current in the third treatment zone, and a pumping current in the fourth treatment zone are all introduced into the calculating section so as to receive a predetermined calculation, thereby making it possible to calculate and then output an oxygen concentration or an air/fuel ratio A/F or an air ratio xcex. Here, the object gas component is NOx whose concentration may be calibrated in accordance with the calculated oxygen concentration or the air/fuel ratio A/F or the air ratio xcex. Although it is preferred that at least the operating section and the amplifier are integrally formed with the gas sensor, such an operating section and an amplifier may also be separated from the gas sensor, but received into a receiver unit comprising a calculating section, displaying and outputting section, and heater operating section.
Moreover, according to the present invention, there is provided a gas analyzer calibrating method for use with a gas analyzer which comprises: a gas sensor which includes a first diffusion rate controlling passage, a first internal space communicated with the first diffusion rate controlling passage, a second diffusion rate controlling passage, a second internal space communicated with the second diffusion rate controlling passage, a third diffusion rate controlling passage, a third internal space communicated with the third diffusion rate controlling passage, and an air introducing duct; wherein the first diffusion rate controlling passage is a passage provided for introducing an object gas containing an object component including a sort of combined oxygen from an external gas existing space to the first internal space under a predetermined diffusion resistance; wherein the first internal space is provided for effecting the combustion of some combustible gases, and is provided with a first electro-chemical pump for adjusting an oxygen partial pressure within the first internal space, so that the internal space constantly contains a sufficient amount of oxygen capable of effecting the combustion of the combustible gases contained in the object gas which has been introduced into the internal space through the first diffusion rate controlling passage; wherein the second diffusion rate controlling passage is a passage provided for introducing the object gas treated in the first internal space to the second internal space under a predetermined diffusion resistance; wherein the second internal space is provided with a second electro-chemical pump which is so constructed that when an amount of oxygen is drawn from the second internal space atmosphere consisting of the object gas introduced into the second internal space through the second diffusion rate controlling passage, the oxygen partial pressure is decreased to a sufficiently low value which does not reduce or decompose the object gas and which is low enough for controlling the oxygen partial pressure in a third treatment zone; wherein the third diffusion rate controlling passage is a passage provided for introducing the object gas treated in the second internal space to the third internal space under a predetermined diffusion resistance; wherein the third internal space is provided with a third electro-chemical pump and a fourth electro-chemical pump on the third diffusion rate controlling passage, the third electro-chemical pump is so constructed that when the oxygen partial pressure of the atmosphere within the second internal space has a value which will not substantially reduce or decompose the object gas, the oxygen partial pressure is controlled to a further lower value which does not bring about any significant influence to the measurement of an amount of an object gas component, whereas the fourth electro-chemical pump is provided to reduce or decompose the object gas component introduced from the second internal space and to draw out an amount of oxygen generated at this moment; wherein the air introducing duct is provided in a manner such that the outside pump electrodes of the first and second electro-chemical pumps are isolated so that these electrodes are not directly exposed to the object gas and that this duct can serve as oxygen sources when oxygen is introduced into the first internal space; the gas analyzer. further comprising: an operating section for operating the electro-chemical pumps provided in the first to third internal spaces of the gas sensor; a calculating section for performing a predetermined calculation on pumping currents flowing into the electro-chemical pumps so as to obtain a concentration value of the object gas component; a displaying/outputting section for displaying a value calculated in the calculating section or for outputting the value as an electric output; and a heater operating section for heating the gas sensor to a predetermined temperature; wherein plural known gas components are used as a standard gas, a pumping current with respect to the standard gas is calibrated as an analytical curve.
Further, according to the calibrating method of the present invention, it is preferred to use a standard gas containing at least H2O or CO2 besides an object gas component as a known object gas component. Preferably, prior to measuring an analytic curve, the gas sensor is heated for a predetermined time to a temperature which is 50xc2x0 C. higher than its working temperature, the gas sensor is then returned to its working temperature, thereby preparing and obtaining an analytical curve with the use of the standard gas. More preferably, prior to measuring an analytic curve, the gas sensor is separated from the operating section, an AC power source is connected to a point between each pair of electrodes in first to three treatment zones, an AC current having a frequency of 1 Hz or more is then caused to flow thereto, subsequently the gas sensor is returned to its driven state, thereby preparing and obtaining an analytical curve with the use of the standard gas.